摘要:

ABSTRACTThe effect of rock fragments and rock fragment cover on the accumulation of airborne dust was examined in a long‐term field experiment in the Negev desert of Israel. Four parameters were studied: pebble eccentricity, pebble size, pebble flattening and cover density. The effect of these parameters on the accumulation of dust on the pebbles, on the accumulation of dust between and underneath the pebbles, and on total dust accumulation (pebbles+interpebble space) was measured separately. Accumulation on the pebbles increased as the pebbles became larger, less flattened and more elongated, and as cover density increased. Accumulation between and underneath the pebbles increased as the pebbles became smaller, more flattened and more elongated, and as cover density increased (although the accumulation area available became smaller). Total dust accumulation increased as the pebbles became smaller, more flattened and more elongated, and as cover density increased.Rock fragments act as a dust trap as dust accumulation on pebble‐covered surfaces is several tens of times larger than dust accumulation on similar but pebble‐free surfaces. However, less than 20% of the initially settled dust is protected from further erosion.In the Negev desert, dust erosion by wind is of the order of 10 times larger than dust erosion by water. The rate of long‐term dust accumulation in the Negev is of the order of 15–30 g.m−2.yr−1. This is considerably lower than the rates proposed elsewhere in the literature, which are based upon over optimistic dust retainmen

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00380.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTA series of wind tunnel tests were carried out to investigate the development of deflation lags in relation to the non‐erodible roughness element concentration. Glass spheres (18 mm in diameter) were placed along the complete length of the wind tunnel working section in regular staggered arrays using three different spacings (d=18, 30 and 60 mm) and completely covered with a 0.27‐mm erodible sand. A pre‐selected free stream velocity above threshold (8m s−1) was established above the surface and the sediment transport measured at 2‐s intervals using a wedge‐shaped trap in which an electronic balance is incorporated. Throughout each test, the emerging lag surface was periodically photographed from above at two locations upwind of the trap. The photographs were electronically scanned and analysed to calculate the lag element coverage and location, as well as mean height and frontal area for each time period. Test results indicate that lag development has a profound effect on both the sediment flux and wind profile characteristics. Initially, there is an increase in sediment flux above that for a rippled sand bed because of increased erosion around and reduced kinetic energy loss in highly elastic collisions with the emerging roughness elements. With further emergence, a dynamic threshold is reached whereupon the sediment flux decreases rapidly, tending towards zero. At this point, the supply of grains to the air stream through fluid drag follows a reduction in aerodynamic roughness and, therefore, surface shearing stress. At least as important is the lesser potential for grain ejection at impact because of reduced momentum imparted from the air stream during saltation. Although recent shear stress partitioning models indicate when particle movement may commence on varying surfaces, our experimental results demonstrate that this partitioning has a further direct bearing upon the saltation

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00381.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTLacustrine deposits are well represented in the lower part of the Late Carboniferous Agua Colorada Formation in the north‐west Sierra de Narváez, Catamarca Province, Argentina. Lake Narváez was one of the several water bodies formed in the region immediately after the Gondwana glaciation.The lacustrine transport system has been divided into three distinct zones: delta, shallow lake and deep lake. Delta progradation proceeded from the ESE. Coarse‐grained delta plain and turbidite delta front deposits suggest that the delta was formed close to the headwaters (‘short‐headed stream delta’type). During periods of high discharge, river mouths acted as bypass zones and fine and very fine sands were transported further into the lake by underflow currents. The clastic material supplied by the deltaic system was partially reworked by wave action. Sands accumulated in unstable conditions at the upper delta front as a consequence of delta progradation. As a result of the addition of clastics in the steep delta front, turbidity currents were formed, spreading their load along the lower delta slope. Deep lacustrine deposits are typically stacked, forming two different kinds of progradational turbidite lobe sequences. Type I lobes were formed in a basinal setting and were probably detached from their feeder systems as a result of sediment‐bypassing in a shallow lake during periods of low lake level. These turbidite lobes are replaced upwards by type II lobes, which were formed on the delta slope during periods of lake level rise that allowed the onset of delta progradation. The presence of highly deformed sandstone bodies suggests rapid depositional rates in a high slope setting, whereas the occurrence of hummocky cross‐stratified sandstones indicates wave reworking of the sands initially emplaced by turbidity currents. Therefore, the inner part of type II lobes was formed above storm wave base. The depositional history of Lake Narváez can be traced through four evolutionary stages: lake transgression, formation of type I lobes, formation of type II lobes and delta progradation. Tectonic activity was probably important at the early stage of lake evolution, but the subsequent depositional history was mainly controlled by fluctuatio

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00382.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTCarbonate pond deposits occur associated with alluvial sediments in Miocene sequences of the Madrid Basin, central Spain. The ponds developed near the basin margins, either in floodplain environments (north) or mud‐flat settings (south). Three main facies assemblages are recognized: (1) floodplain/mud‐flat, (2) palaeosols and (3) pond deposits.In the northern part of the basin, ponds developed on the floodplain of terminal fluvial systems. The floodplain facies are typically red mudstones with interbedded sandstones and siltstones. Palaeosols associated with the ponds show a pedofacies relationship, the maturity of soils increasing with distance from the main channel. Carbonate pond deposits consist mainly of limestones, which display typical ‘palustrine’features. The formation and further accumulation of carbonate in the ponds took place in periods of reduced clastic sediment input and it is suggested that recharge into the pond areas was mainly from groundwater.In the south, ponds developed on mud‐flats located between sheet‐flood‐dominated alluvial fans and evaporite lakes. Mud‐flat facies consist of red mudstone that exhibits evidence of progressive soil development near both edges and beneath the carbonate pond lenses. Carbonate in the ponds is mainly dolomite and comprises two subfacies, mottled and laminated dolomicrites. This mineralogy, together with the presence of gypsum crusts below and in the lower part of the carbonate body, suggests higher evaporation rates and/or more saline waters filling the ponds in this part of the basin.In spite of differences in depositional setting and, to some extent, climatic conditions between the two areas of the basin, both facies associations and the sequential arrangement of the ponds show strong similarities that allow the proposal of a facies model for carbonate pond deposits related to semi‐arid alluvial systems. The sequences recognized from the pond deposits record a set of facies clearly different to those forming in swampy lakes associated with many permanent fluvial systems developed in mo

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00383.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTThe propagation of and the deposition from a turbulent gravity current generated by the release of a finite volume of a dense particle suspension is described by a box model. The approximate model consists of a set of simple equations, a predetermined, depth‐dependent leading boundary condition and one experimentally determined parameter describing the trailing boundary condition. It yields predictions that agree well with existing laboratory observations and more complex theoretical models of non‐eroding, non‐entraining, suspension‐driven flows on horizontal surfaces.The essential features of gravity‐surge behaviour have been observed and are captured accurately by the box model. These include the increased rate of downstream loss of flow momentum with increased particle setting velocity, the existence of maxima in the thickness of proximal deposits, and the downstream thinning of distal deposits. Our approximation for the final run‐out distance, xr, of a surge in deep water is given by xr3(g'oq3o/w2s)1/5, where g'ois the initial reduced gravity of the surge, qothe initial two‐dimensional volume, and wsthe average settling velocity of the particles in the suspension. A characteristic thickness of the resulting deposit is given by φoqo/xr'where øois the initial volumetric fraction of sediment suspended in the surge.Our analysis provides additional insight into other features of gravity‐surge dynamics and deposits, including the potential for the thickening of currents with time, the maintenance of inertial conditions and the potential for strong feedback in the sorting of particle sizes in the downstream direction at travel distances approaching xr. Box‐model approximations for the evolution of gravity surges thus provide a useful starting point for analyses of some naturally occurring turbidity surges

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00384.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTPlatform carbonates of the Upper Triassic Dachstein Limestone in Naszály Hill have been karstified extensively over the past 200 million years. They provide an excellent example of polyphase karstic diagenesis that is probably typical of many subaerially exposed carbonate sequences.Seven karstic phases are recognized in the area, each of which include polyphase karstic events. The first karst phase was associated with the Löfer cycles. Meteoric waters caused dissolution; enlarged fractures and cavities, were filled by marine and/or vadose silts and cement.The second karst phase was caused by local tectonic movements. Bedding‐plane‐controlled phreatic caves were formed, and filled by silts.The third karst phase lasted from the end of the Triassic to the Eocene. This was a regional, multiphase karstic event related to younger composite unconformities. Bauxitic fill is the most characteristic product of this phase. The karst terrain reached its mature or senile stage with very little porosity.Narrow veins and floating rafts of white calcite marks karst phase 4, which resulted from hydrothermal activity associated with Palaeogene magmatism. The early Rupelian phase of Alpine uplift caused large‐scale rejuvenation of the former karst terrain (karst phase 5). Subsequently Naszály Hill was buried as an area of juvenile karst with significant porosity. A second period of hydrothermal activity in the area (karst phase 6) was induced by Miocene volcanism, which resulted in wide, pale green calcite veins. Finally karst phase 7 was of tectonic origin. Following the most recent, Miocene uplift of the Naszály Hill, the carbonates have again become the site of vadose karst de

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00385.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTDetailed measurements of flow velocity and its turbulent fluctuation were obtained over fixed, two‐dimensional dunes in a laboratory channel. Laser Doppler anemometry was used to measure the downstream and vertical components of velocity at more than 1800 points over one dune wavelength. The density of the sampling grid allowed construction of a unique set of contour maps for all mean flow and turbulence parameters, which are assessed using higher moment measures and quadrant analysis. These flow field maps illustrate that: (1) the time‐averaged downstream and vertical velocities agree well with previous studies of quasi‐equilibrium flow over fixed and mobile bedforms and show a remarkable symmetry from crest to crest; (2) the maximum root‐mean‐square (RMS) of the downstream velocity values occur at and just downstream of flow reattachment and within the flow separation cell; (3) the maximum vertical RMS values occur within and above the zone of flow separation along the shear layer and this zone advects and diffuses downstream, extending almost to the next crest; (4) positive downstream skewness values occur within the separation cell, whereas positive vertical skewness values are restricted to the shear layer; (5) the highest Reynolds stresses are located within the zone of flow separation and along the shear layer; (6) high‐magnitude, high‐frequency quadrant‐2 events (‘ejections’) are concentrated along the shear layer (Kelvin‐Helmholtz instabilities) and dominate the contribution to the local Reynolds stress; and (7) high‐magnitude, high‐frequency quadrant‐4 events occur bounding the separation zone, near reattachment and close to the dune crest, and are significant contributors to the local Reynolds stress at each location. These data demonstrate that the turbulence structure associated with dunes is controlled intrinsically by the formation, magnitude and downstream extent of the flow separation zone and resultant shear layer. Furthermore, the origin of dune‐related macroturbulence lies in the dynamics of the shear layer rather than classical turbulent boundary layer bursting. The fluid dynamic distinction between dunes and ripples is reasoned to be linked to the velocity differential across the shear layer and hence the magnitude of the Kelvin‐Helmholtz instabilities, which are both greater for dunes than ripples. These instabilities control the local flow and turbulence structure and dictate the modes of sediment entrai

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00386.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTThis paper reviews some aspects of wind tunnel experiments on sand‐transporting winds. It follows previous papers that have discussed the influence of the outer region of the boundary layer on wind velocity measurements. This influence was quantified with the use Coles’Wake function. In this paper this correction is applied to six previously described wind velocity profiles. An attempt is made to calculate the profile parameterIIfrom these measurements. The values found were not consistent with the expectedII, which was determined by Coles for clean air flow. This value (II=0.55) was assumed to be valid in previous analyses for sand‐transporting winds. Evidence for a mutual dependency of friction velocity and profile parameter is presented and the difficulty in determining u. is pointed out. It is suggested that the constant stress region of the boundary layer should be kept large enough for measurements when Coles’Wake function is not to be used in the data a

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00387.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00388.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00389.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY